Siren

Sirens are women with mystical, unknown backgrounds and incredible powers, and are distinguishable by the tattoos running down one half of their body. Only six Sirens can exist at any given time, as revealed by Handsome Jack. Three are known: Lilith, Maya, and Angel, and a Siren-human hybrid. The Crimson Lance leader Commandant Steele was the fourth Siren until her death.

Sirens
There are reportedly six Sirens in existence, and only six are possible, although Matt was a half-Siren male. Lilith is one of them, and part of the reason that she was on Pandora involved searching for another whom she believes to be on the planet. The Siren she sought was most probably Commandant Steele, due to Steele being the only known Siren besides Lilith at the time. A third Siren to appear was Angel, an enigmatic character who was later revealed to be a Siren. The full extent of Angel's abilities were unknown, however she makes references to performing a "phaseshift" when hacking technology. The final known Siren was Maya.

Matt was a half-Siren, not necessarily a full one, although governed by similar rules to other Sirens. He explained his existence as a computer rounding down 6.5 to 6, and did not count it as a more than 6, thus allowing for him to exist. He was chosen by the Enlightener to become a half-Siren, so it is more likely that he exists because the Enlightener can break his own rules. He also does not have any Siren tattoos.

The tattoos are a side-effect of being a Siren. The tattoos of all known and suspected sirens, however, only appear on one half of the body. Seemingly they channel their powers through the arm that possess the tattoos, which is often seen when Maya uses her power, Phaselock, but this could be because her right arm is always holding a weapon. The tattoos appear to cover them from foot to scalp, as seen with Maya. Whether or not it is the same for other known Sirens is unconfirmed.

Sirens thrive on Eridium ore, a mineral that became abundant on Pandora after the opening of the Vault of the Destroyer. Patricia Tannis suggested that, because of their relationship with Eridium, there should be a connection between Sirens, the Vaults, and the Eridians. This connection is not between Eridium and the individual Siren, as Angel mentioned that Maya has no previous connection to Eridium in ECHO recordings.

Siren powers are enhanced through the use of Eridium. This was seen when Lilith used Eridium to enhance her phasewalk ability and heal herself. This was also seen when Handsome Jack uses Eridium to increase Angel's powers. Over-consumption of the material will have negative side effects. Lilith shows signs of addiction to the substance. Angel, being fed enormous amounts over a long period of time, eventually required a constant feed in order to maintain life function. She died shortly after her Eridium supply was cut off.

Abilities
Each Siren has an ability unique to themselves, albeit sharing similar characteristics. Lilith had the ability to Phasewalk through another dimension for a short period of time. This ability was enhanced through the use of Eridium to teleport others as well as herself, and through the use of a high concentration of Eridium to amplify this trait she has also been able to phase the entire city of Sanctuary.

Maya had the ability to Phaselock enemies, rendering them completely immobile in a bubble made of energy from another dimension.

Angel uses an ability that she refers to as Phaseshift. The ability allows her to change the way her environment looks. This was shown when she phased one of the Control Core chambers away in the Bunker. Later, during the fight in the main chamber, Angel was able to spawn ammo boxes. By being connected to all of Pandoras' datastreams, Angel was able to open doors and hack computers or consoles, like the Catch-A-Ride station. She also claimed to be able to "look forward and backward along the timeline".

Other known traits include being able to power themselves up physically. Both Lilith and Maya can deliver empowered melee strikes in close quarters that can match or, depending on skill investments, exceed the damage inflicted by a number of the other characters' melee weapon strikes. Lilith's Phase Strike skill makes a notable feature of this talent.

Both Lilith and Angel have demonstrated the ability to charge a Vault key, thereby providing the power to activate the Warrior. Steele has also demonstrated an understanding of Vault keys, as she has been seen assembling one to open the Vault of the Destroyer. Maya is implied to notably lack this ability by Angel. However, no explanation is given for this.

Physics
In reality, Sirens are connected to a dimension known as the Infinite, which gives them their powers. They exist as the means of balancing the equation between the Infinite and Midgard (the Earth dimension), as when the Eridians left Midgard, they left traces of Infinite energy behind that would have damaged the dimension if not dealt with. To combat this, they created Sirens, forming six as six was the exact amount required to soak up the excess Infinite energy and thus maintain the dimension.

Most Sirens are able to power themselves up using Eridium, and this is because Eridium is calcified Infinite energy, and as a result can be used by Infinite energy conductors and Infinite-compatible technology, such as Vault Keys.

The Infinite grants Sirens their powers through the myriad of ways it can be used: Sirens' tattoos are branding of their connection to the Infinite, and the patterning is actually the Eridian language, although not all Sirens were branded with these tattoos and it is optional if they will receive, according to the will of the Enlightener.
 * Lilith enters a half-realm between Midgard and the Infinite when she enters her Phasewalk, allowing her teleport by moving through space-time.
 * Maya's Phaselock bubble is comprised of Infinite energy, specifically the forcefield end of the spectrum.
 * Matt draws from the Infinite to control time in his Chronokinesis, and releases it in other forms to maintain control over the timestream.